The present invention relates to the field of debris extraction. More specifically, the present invention relates to the field of debris extraction from liquid.
In nuclear-powered energy-generation facilities, there is a need to remove debris and foreign objects from tanks of potentially radioactive water.
During reactor plant operation, activated corrosion products along with fission fragments are released into the reactor coolant system. These are the sources of highly radioactive particles encountered during refueling operations. In the case of fuel failure, portions of fuel cladding and structural materials may also be encountered. Fuel-element cladding is subjected to varying amounts of heat, which heat brings about various chemical changes in the cladding. Since the water usually contains iron ions leeched from structural members, these changes typically involve the conversion of the cladding into oxides and/or ferroalloys of the cladding material, (e.g., zirconium oxide and/or various zirconium ferroalloys if the cladding is zirconium).
Such oxides and ferroalloys are often significantly softer than the original cladding material. This softer material tends to flake or slough off, resulting in debris within the containment tank. This irradiated or xe2x80x9chotxe2x80x9d debris requires removal on a regular basis, typically during refueling. Since each particle of this debris may be so small as to border on the limits of visibility, especially underwater, this debris may be considered as composed of miniscule or xe2x80x9cmicroxe2x80x9d objects.
In extreme cases, the debris may contain fragments of fuel and/or fuel byproducts (i.e., uranium, plutonium, and/or their oxides). Such debris is considered extremely hot and/or toxic, and must be removed with the utmost care. Since such fragments, too, are micro objects, the interests of safety assert that the entirety of the micro-object debris is considered to be of such objects whenever any of the debris may be so considered. This increases significantly the care exercised during the debris-removal procedure.
Additionally, a large (i.e., xe2x80x9cmacroxe2x80x9d) object may occasionally be discovered in the containment tank. Such macro objects may be objects dropped into the tank by personnel (e.g., a coin or ring), may be a part of the facility itself (e.g., a fastener), or other foreign materials. Any such macro object is typically retrieved from the tank and identified.
A partially subaqueous vacuum cleaner is the prior-art instrument of choice to effect removal of micro objects. Such a partially subaqueous vacuum cleaner typically utilizes a heavy, non-submersible motor and pump. A long hose connects a cleaning head with the pump. Such long hoses tend to collect debris. With potentially radioactive debris, there exists a very real problem of the hose itself becoming radioactive.
Because of size, shape, and/or mass, however, macro-object debris may often be beyond the retrieval abilities of a typical subaqueous vacuum cleaner, due in part to the strength of the flow as hampered by a long hose, etc. For example, a vacuum cleaner may require more than available power to retrieve a flat object, such as a dime, as such an object may be pressure-bound to the tank bottom by the very water flow generated by the vacuum. Thus, an alternative apparatus, such as a grabber, may be required to retrieve some macro objects.
An additional problem arises in that such macro objects are desirably inspected after retrieval. Indeed, such an inspection may be required by established safety procedures. A typical prior-art vacuum cleaner uses a relatively large fiberglass filter. The contents of such a filter (i.e., the debris entrapped within the filter by the cleaning operation) are not readily examined. Indeed, were a foreign macro object to be entrapped in such a filter, the filter would be removed from the tank, taken to a safe inspection area, and cut open to reveal the foreign object. Therefore, the use an alternative retrieval apparatus (e.g., a grabber) for macro-object debris becomes an effective requirement.
Even for micro-object debris, cleaning is not completely straightforward. Unlike a swimming-pool structure, a containment tank has many angles, corners, alcoves, canals, wells, and other structures and certified spaces that must be navigated during the cleaning process. This severely limits the size and shape of the actual cleaning portions of a subaqueous cleaning apparatus. Additional difficulties are encountered when a typical hose-connected cleaning head must be maneuvered around and through these structures.
Another problem arises in that the containment tank is deep, especially in comparison to a swimming pool. Therefore, whatever type of vacuum is used is desirably able to be both operated and controlled while at an appropriate depth by an operator outside the tank. A common device to effect such control is a long pole. However, a trailing hose containing moving water provides a continuous opposition to easy control, especially when connected to the pole at a considerable distance from the operator.
The debris collected by a subaqueous vacuum is potentially radioactive, often much more radioactive than the water in which it is immersed. A problem arises in removal of such debris. Conventionally, such debris is collected in a filter, typically of fiberglass. This contaminated filter must then be removed from the containment tank. In current practices, such a filter is disconnected from the rest of the vacuum and encased in a shielded cask while under water. The cask is then removed from the water and moved to a disposal area. The problem arises in that, since the cask contains radioactive debris, it must be heavily shielded. Such a cask is exceedingly heavy and requires the use of an overhead crane. This poses health and safety risks to the personnel involved in the filter removal operation.
Another factor in all aspects of the debris removal process is time. The containment tank is typically cleaned while the reactor is off-line. Since the cost of the reactor being off-line may be several tens of thousands of dollars per hour, the cleaning process is preferably only a small portion of the refueling process. To wit, the reactor is placed off-line, the core is defueled, the tank and all components are cleaned, the core is refueled, and the reactor is placed on-line. As the defueling and refueling processes are substantially fixed in time, it is most desirable that the cleaning process be accomplished as quickly as possible.
Many structures other than a containment tank require cleaning. A significant number of these structures may be cleaned while the reactor is on-line. Being on-line does not, however, eliminate the need for a timely, efficient, and safe cleaning operation.
With a conventional, fiberglass-filter vacuum-cleaning system, a considerable period of time is consumed introducing the system to the tank, and an even greater period of time is consumed extracting first the filter, then the system from the tank. This set-up and break-down time is in addition to any time spent actually cleaning the tank.
The present discussion is primarily concerned with the problems encountered cleaning a tank of potentially radioactive water in a nuclear-powered energy-generation facility. Those skilled in the art will appreciate that a similar discussion may be made about the cleaning of tanks or pools of other types of liquids in other type of facilities whenever the liquid is contained in confined or intricate spaces and/or hazardous to humans upon contact.
Accordingly, it is an advantage of the present invention that a liquid-tank debris-extraction system and method of operation thereof is provided.
It is another advantage of the present invention that a submersible, self-contained vacuum unit is provided that may be used to clean debris from a tank of liquid.
It is another advantage of the present invention that a submersible vacuum unit is provided having a detachably coupled filtration unit where the filtration unit may be detached while under water.
It is another advantage of the present invention that a submersible vacuum unit is provided wherein a reverse flow of liquid extracts debris entrapped in a filtration unit.
The above and other advantages of the present invention are carried out in one form by a system for the extraction of debris from a tank of liquid. The system has a vacuum unit configured to be completely submersible in the liquid and containing a reversible motor, a pump coupled to the motor and a filtration unit removably coupled to the pump and incorporating a filter configured to entrap the debris. The system also has a control unit electrically coupled to the motor and configured to determine a rotational direction of the motor from outside the tank. The pump passes a portion of the liquid through the filtration unit as a forward flow when the control unit causes the motor to operate in a forward rotational direction. The pump passes a portion of the liquid through the filtration unit as a reverse flow when the control unit causes the motor to operate in a reverse rotational direction.
The above and other advantages of the present invention are carried out in another form by a method of extracting an object of debris from a tank of potentially radioactive water in a nuclear-powered energy-generation facility. A reversible motor, a pump coupled to the motor, and a filter housing removably coupled to the pump are immersed into the tank. A filter is incorporated within the filter housing, the motor and pump are driven in one of a forward and a reverse rotational direction via a control unit outside of the tank. One of a forward and a reverse flow of water, respectively, is passed through the pump, the filter housing, and the filter.